The demand for greater numbers of TV channels in cable TV systems is increasing A typical cable TV system includes 60 analog TV channels, expandable in capacity to 80 or more channels.
In one contemplated information superhighway system, 132 or more digital channel, data channel, and telephony signals may be required in a cable TV system. Digital video, data and telephony signals are modulated on analog RF carriers. Digital video systems provide video on demand services.
Fiber optic transport systems are currently contemplated for carrying information from a cable TV headend center or from a telephone central office, to a fiber optics service node near subscribers. Coaxial cables may then be used for distribution of information to subscribers. A fiber optics system relies upon optical fiber as a transmission media. In such systems, a laser may be used to convert RF signals into optical signals, so that the signals can be transmitted in an optical fiber.
In order to ensure high fidelity delivery of information, a laser's transfer function for converting RF signals to optical signals must be linear. If the transfer function is not linear, harmonic generation, inter-modulation, and multiple beating between parallel channels will significantly reduce cable TV picture quality Unfortunately, the linearity of the transfer function of a laser is generally not sufficient to avoid distortion.
Methods for linearization of laser transfer functions have been studied for years. Such methods are generally complicated and costly to implement. In particular, optical feed forward techniques for linearization have been studied A great deal of laser distortion is due to operation under conditions of saturation. During operation in saturation, optical signals from a laser are reduced in proportion to RF signal magnitude, when the signal level of the laser is high. According to one known approach, a second laser is employed according to a known feed forward technique to generate a pre-distorted optical signal. A pre-distorted optical signal is then added to a main optical signal to compensate for laser saturation. Such compensation works, but there are three major disadvantages to this approach. First, the wavelengths of the two lasers have to be matched. If there is mismatch between the lasers due to fiber dispersion, for example, the two laser signals will travel at different speeds in the fiber, limiting linearization. Second, the output of the pre-distortion laser and the main laser change independently over time, causing undesirable linearization variations. Third, the use of multiple lasers is expensive, and the circuitry to implement optical feed-forward is costly. It is accordingly desirable that a pre-distortion solution be developed which is simple, low cost, and easy to implement.
FIG. 1 according to the published paper shows the linearity improvement of a transfer function of a microwave amplifier from the paper titled "Pre-distortion Nonlinear Compensator for Microwave SSB-AM System" published by Toshio Nojima et. al. in Electronics and Communications in Japan, Vol. 67-B, No. 5, 1984. According to FIG. 1, distortion reduction depends on amplitude and phase match between a pre-distortion signal and the distortion signal of an amplifier transfer function. Amplitude adjustment and phase adjustment circuits need to be added to minimize amplitude error and phase error of a pre-distorted signal and a distortion signal of a amplifier transfer function. Amplitude tuning and phase tuning can be employed according to the paper for narrow-band applications. In a broad-band system according to the present invention, bandwidth is from 50 MHz to 750 MHz, or even up to 1000 MHz. Such a frequency span covers 4 to 6 octaves As is well known, distortion is frequency dependent. Frequency dependent amplitude adjustment and phase adjustment of a pre-distorted signal are needed to ensure significant distortion reduction for an whole bandwidth.